I recently started playing Candy Crush Friends Saga, and I already really like how it handles event levels. Instead of replacing the last level of an episode, event levels have their own progression that is separate from the main game. This means they are optional and can be played at one's own leisure.
I think Candy Crush Saga should do something similar to Candy Crush Friends Saga with its expeditions. Move expeditions into their own event that runs in parallel, and allow players to go through the event levels as they please. Put a really nice reward at the very end and/or at specific milestones. This would leave the main game untouched and provide an optional challenge to those who want it. Additionally, make it possible to beat the event levels more than once. It would make helping other players easier.
There are a few benefits to this:
(1) It's easier to help people who are stuck on a level. With expeditions' current functionality, the last level of an episode is completely overwritten, and the new level depends on how far people are in the expedition. This means that, for example, my level 6935 is very different from someone else's level 6935, and a third person's level 6935 is probably very different as well. Considering that players have Bubblegum Season, Music Season, or no season at all, helping players at the end of an episode is often very troublesome.
If expeditions are moved into their own side event, then all ambiguity is gone for levels at the end of an episode. Making event levels beatable multiple times also makes it easier for people to help others with the event levels.
(2) It reduces player frustration. One of the most common complaints about expeditions is that their levels are way too hard. Right now, they are meant to be "boss" levels. Moving expeditions into their own side event would incentivize the creation of easier event levels at first, and the fact that they wouldn't be "boss" levels anymore means that they would be less pay-to-pass. Players who don't want to deal with the really hard event levels can just ignore them. Those who want to embrace the challenge can beat the event levels on their own terms. So far, from what I've experienced in Candy Crush Friends Saga, this is how event levels work.
(3) It reduces monotony at all levels of the game. Expeditions are a great way to give players a taste of more advanced game elements (especially the case with the current Music Season's color clusters and rainbow rapids). I'm only on level 36 of Candy Crush Friends Saga, but the events have introduced me to hazelnut crunches, pop rocks, licorice links, and more later-game blockers. That's awesome! I love being able to take a break from the simple early-game levels and dip my toes into some much more complex levels with advanced elements. I don't think I'm alone here.
I really want to play the current Music Season's levels, but I don't want them to be forced pay-to-pass levels that overwrite existing levels. Again, I don't think that I'm alone here. With how expeditions are currently set up in CCS, the vast majority of the game is spent in the monotony of passing new levels that very slowly add new elements to the game. Moving expeditions to their own side event would mean players can experience unique levels and more advanced elements to take a break from the main progression.
In Candy Crush Saga, the first 4,000 levels or so are a huge slog. New elements usually aren't very interesting, and they are few and far between. Optional event levels on the side would be a breath of fresh air. For players at the end of the game, optional event levels would temporarily free them from the endless cycle of the Star Tournament.
By making expeditions optional and separate from the main game, they would give players a lot more to do.